This invention relates generally to steam generators. More particularly, the present invention relates to waste-heat steam generators or boilers which are heated by means of hot exhaust gases.
Such steam generators are primarily fed with hot exhaust gases from energy and/or process technology systems, and they often comprise a plurality of water-side pipe sections or circuits that not only have varying geometries but also have widely divergent heat capacities. For this reason, it is often necessary to control the distribution of the circulating water to individual pipe sections or circuits, for example with the aid of flow restrictors.
In the case of prior-art mechanically-circulated steam generators, the distribution of the circulating water to individual water-side pipe sections is controlled by means of orifice restrictors installed at the inlet to the individual heating surface coils or pipe sections (La Mont system). The pressure difference caused by the individual pipe sections and the orifice restrictors must be overcome with the aid of a circulating pump.
Controlling the circulating water in a gravity-circulation steam generator is a difficult problem since these steam generators generally lack sufficient pressure difference to allow orifice restrictors to be installed. The available pressure difference in the individual pipe sections or circuits is predetermined by the intensity of heating, the height difference and the pressure loss in the individual pipe sections. In this case, the installation of nozzle or orifice restrictors to improve the distribution of water is based on the idea of restricting the flow of water in the pipe sections that have good circulation in order to increase the circulation of water in the low-circulation pipe sections by means of a lower frictional pressure loss in the common descending and ascending lines. The total rate of circulation in the system is often greatly reduced in a disadvantageous manner, and only a modest improvement can be achieved for the affected pipe section-in other words, the weakly circulating pipe section.
The object of the invention is to provide a steam generator in which the water circulation in the individual pipe sections/circuits can be distributed more effectively without having a significant adverse effect on the total water circulation rate in the system.
The solution offered by the invention provides a steam generator that has the following advantages. It can distribute the water circulation rates in each pipe section or circuit as needed by increasing the pressure in the pipe section or sections in which an increase in the circulation rate is necessary or desired, without causing an additional pressure loss due to friction in the pipe section which does not require a pressure increase-in other words this measure can
a) compensate for the lack of upward flow in a pipe section or in a plurality of pipe sections,
b) more successfully overcome an inherently high pressure loss in a pipe section so that it is more closely matched to the other pipe sections or is matched to them as completely as possible,
c) supply an evaporator device that is located within a steam generator and that has relatively high cooling requirements-for example, an end plate or a tube plate in a firetube boiler-with a relatively high quantity of cooling water,
d) the pressure increase in the pipe section or in the pipe sections in which an increase in the circulation rate is required can be achieved without the use of an additional pump.
In a preferred embodiment of the invention, the venturi device comprises a venturi nozzle inserted in the descending pipe of a water/steam circuit. This makes it easy to configure the descending pipe with a standardized, commercially available nozzle, for example an EN ISO 5167-1 venturi nozzle.
In a preferred embodiment of the invention, the venturi device comprises a descending pipe line in the form of a venturi pipe. Thus, the venturi device is completely integrated in the descending line, and, if desired, it can be made of the same material and from a single piece.
Preferably, the steam generator of the invention is operated under natural convection flow. In this mode, one or more water/steam circuits that, for various reasons, has/have a weaker rate of circulation compared to a different or additional circuits can be operated at an increased water circulation rate without having to resort to additional pumps and consequently increasing capital spending, operating, and maintenance costs.
It is also advantageous to operate the steam generator of the invention with forced circulation. In this mode, one or more water/steam circuits that, for various reasons has/have a weaker rate of circulation compared to a different or additional circuits, can be operated at an increased water circulation rate.
In one preferred embodiment of the invention, the ratio of the inside diameter d of the venturi nozzle device at its narrowest cross section to the inside diameter D of the descending pipe is between 1.0 and 0.01. This embodiment ensures that the effect of an increased water flow rate is established in the circuit whose inlet is located in the diffuser-shaped outlet of the venturi nozzle device. Examples of the invention are illustrated in greater detail below based on the drawings and the description.